@younesouhbi You only have to sign up to one instance, because instances are connected and you can interact with people from other instances (like me! Hi from cybre.space!) already.
That being said, different, and especially smaller instances do offer a slightly different view into the fediverse, so if you want to discover more, or try what a different place feels like, making a second account somewhere else might be fun.
You can follow people from other instances with your one account! They all connect!*
*Sometimes instances are full of jerks and various admins say "wow, that place is full of jerks", then make it tougher for their users to get hassled by that instance.
@younesouhbi being registered on one instance is essentially all you need to talk to people on any other instance. it's kind of like email services in that regard, if that makes any sense!
the difference you'd get out of registering for another instance is mostly having a different Local timeline (and, of course, different admins, possibly a different Code of Conduct), which you might look for as you hang around the instance you're already on & get a sense of what kind of neighbors you'd like to have. :)
Sign up once and you're set. You can search for and follow people on other instances.
People who share an instance have the option for the greatest privacy. They can opt to send messages that won't leave the instance. But that's sort of contrary to the typical purpose of posting. It might make sense for an organization that wants private discussions.